Control of Stimuli Sensitivity in pH-Switchable LCST/UCST-Type Thermosensitive Dendrimers by Changing the Dendrimer Structure.

Stimuli-sensitive materials, such as pH- and temperature-responsive polymers, are useful as smart materials. Phenylalanine (Phe)-modified polyamidoamine (PAMAM) dendrimers with succinic acid termini, PAMAM-Phe-Suc, have been reported as unique pH-switchable lower critical solution temperature (LCST)-/upper critical solution temperature (UCST)-type thermosensitive polymers. Regulating the phase transition behavior of dendrimers is important for their applications. This study investigated the relationship between the dendrimer structure and stimuli sensitivity. Phe-modified PAMAM dendrimers with cyclohexanedicarboxylate termini (PAMAM-Phe-CHex) and sulfonate termini (PAMAM-Phe-SO3Na) were synthesized. The temperature-dependent transmittance of these aqueous dendrimer solutions was examined at various pH values. PAMAM-Phe-CHex with Phe at all termini (PAMAM-Phe64-CHex) demonstrated a broad UCST-like phase transition at pH 7.0 but lacked an LCST-type phase transition. PAMAM-Phe-CHex with ≤ 27 Phe residues showed both LCST- and UCST-like phase transitions at different pH values, but the phase transition was broad. PAMAM-Phe-SO3Na showed both LCST- and UCST-type phase transitions at different pH values, and the transition temperature increased as the bound Phe number decreased. Thus, the phase transition behavior of PAMAM-Phe-SO3Na dendrimers can be regulated by varying the Phe/PAMAM ratios.

Carboxy-terminal dendrimers with phenylalanine for a pH-sensitive delivery system into immune cells including T cells.

Although T cells play important roles in various immune reactions, there are only a few reports on delivery systems into T cells. Our previous study showed that carboxy-terminal phenylalanine (Phe)-modified polyamidoamine (PAMAM) dendrimers have both temperature- and pH-sensitive properties, which are affected by the chemical structure. The self-assembled structures of Phe, observed in phenylketonuria, enhance the protein aggregation, the association with the cell membrane and the membrane permeability. In this study, we applied the Phe-modified dendrimers to a pH-sensitive drug delivery system into T cells. Dendrimers with different amino acids and acid anhydrides were synthesized, and their pH-responsive association with T cells and their subsets was investigated. The dendrimers modified with Phe and cyclohexanedicarboxylic acid (CHex) showed higher uptake into various cells, including Jurkat cells, CD3+ T cells, CD3 + CD4+ helper T cells and CD3 + CD8+ killer T cells. These dendrimers were internalized into T cells via endocytosis, and their cellular uptake was enhanced under weak acidic conditions (pH 6.5). Our results showed that Phe- and CHex-modified dendrimers have a delivery potential to T cells and their subsets, which may be useful for cancer immunotherapy.

pH-Switchable LCST/UCST-type thermosensitive behaviors of phenylalanine-modified zwitterionic dendrimers.

Thermosensitive polymers are useful as intelligent materials. Dendrimers have well-defined structures, which can work as multifunctional polymers. In this study, we designed and synthesized various phenylalanine (Phe)-modified zwitterionic dendrimers as pH- and thermo-sensitive polymers. First, polyamidoamine (PAMAM) dendrimers were modified with Phe and succinic anhydride (Suc) to prepare carboxy-terminal Phe-modified dendrimers (PAMAM-Suc-Phe and PAMAM-Phe-Suc). Both these dendrimers showed upper critical solution temperature (UCST)-type thermosensitivity. Interestingly, PAMAM-Phe-Suc demonstrated lower critical solution temperature (LCST)-type thermosensitivity at lower pH, but PAMAM-Suc-Phe did not. This indicates that PAMAM-Phe-Suc can switch LCST/UCST-type thermosensitivity according to the solution's pH. PAMAM-Phe-SO3Na with sulfonic acid termini also demonstrated LCST/UCST-type thermosensitivity switched by pH, with a higher sensitivity than PAMAM-Phe-Suc. Coacervation occurred during the phase separation. The quaternized dendrimers (QPAMAM-Phe-Suc and QPAMAM-Phe-SO3Na) and dendrimers conjugating isoleucine or 4-(amino methyl)benzoic acid did not show the unique thermosensitive properties, indicating that the tertiary amines in the dendrimer core and the Phe residues at the termini are indispensable. PAMAM-Phe-SO3Na could separate a model compound (rose bengal) from an aqueous solution because of its encapsulation ability. This is the first report of pH-switchable LCST/UCST-type thermosensitive dendrimers.

Dual pH-sensitive and UCST-type thermosensitive dendrimers: phenylalanine-modified polyamidoamine dendrimers with carboxyl termini.

Dendrimers are unique polymers with well-defined structures, and are useful as functional unimolecular nanoparticles. Previous reports have shown that polyamidoamine (PAMAM) dendrimers modified with hydrophobic molecules, such as amino-terminal phenylalanine (Phe), are thermosensitive at high pH. In the present study, we designed carboxyl-terminal Phe-modified PAMAM dendrimers that are thermosensitive under acidic conditions. We reacted an amino-terminal PAMAM dendrimer with various acid anhydrides, such as succinic anhydride, cyclohexanedicarboxylic anhydride, and phthalic anhydride, prior to the reaction with Phe. Interestingly, the amino-terminal Phe-modified PAMAM dendrimers exhibited LCST (lower critical solution temperature)-type thermosensitivity at approximately pH 7, but the carboxyl-terminal Phe-modified dendrimers exhibited UCST (upper critical solution temperature)-type thermosensitivity in acidic solutions. Temperature sensitivity was dependent on both pH and the anhydride modifier. We were able to separate rose bengal (a model compound) from aqueous solutions of the carboxyl-terminal Phe-modified dendrimer at low pH.